This application is directed to a more economical method of producing a phosphoric acid with reduced tendency to form detrimental solids (e.g., sludge) during shipment or storage or upon conversion, as by ammoniation, to a liquid fertilizer. The invention is especially useful in the production of superphosphoric acid (hereinafter, sometimes, "SPA") generally having a P.sub.2 O.sub.5 analysis in the range of about 68 to 72 weight percent.
Solids-forming impurities are usually removed from phosphoric acids produced by dihydrate or hemi-hydrate phosphoric acid processes before final evaporation to produce superphosphoric acid, because the high viscosity of SPA hinders settling of solids and makes filtration or extraction of solid impurities difficult, although SPA can be heated to elevated temperatures to decrease its viscosity and make it amenable to filtration.
Purification of phosphoric acid usually results in the loss of some phosphate values, that is P.sub.2 O.sub.5 values. The present invention reduces the degree of phosphoric acid purification required for SPA manufacture; and, as a consequence, diminishes the loss of P.sub.2 O.sub.5 values in producing SPA as compared to conventional methods of producing SPA. The invention also decreases the consumption of filter aid such as diatomaceous earth, during purification by aging and filtration.
Phosphoric acid produced by hemihydrate phosphoric acid processes, especially a low sulfate hemihydrate process (i.e. that is a process producing phosphoric acid of about 38-44% P.sub.2 O.sub.5 with a sulfate ion concentration of less than about three percent (3%)), is sometimes not as readily purified by aging and filtration as phosphoric acid produced by conventional dihydrate phosphoric acid processes due to formation of fine magnesium-containing solids which can pass-through the filter aid on a rotary vacuum filter.
Moreover, when a phosphoric acid produced by a hemihydrate phosphoric acid process (hereinafter sometimes called "hemi-acid" or "hemihydrate process acid") is treated by prior art methods for precipitating and removing solids-forming impurities, some of the precipitate solids are very finely divided and can pass through filters, such as the diatomaceous earth filter disclosed in U.S. Pat. No. 4,235,854.
The purification of phosphoric acid produced by a low sulfate hemihydrate phosphoric acid process can be enhanced by the addition of a precipitant containing calcium and fluoride and; usually an additional sulfate ion, such as by the addition of sulfuric acid. The addition of a precipitant to hemi-acid comprising calcium and fluoride, as disclosed in the U.S. Pat. No. 4,136,199, and U.S. Pat. No. 4,243,643, especially with the addition of sulfuric acid, can be used to form larger precipitated solid particles which are filterable. The preferred precipitant is the sludge produced by the neutralization process of U.S. Pat. No. 4,171,342.
Low sulfate hemihydrate phosphoric acid processes have the economic advantage of consuming less sulfuric acid than other hemihydrate phosphoric acid processes. Accordingly, the addition of sulfate ion to phosphoric acid produced by a low sulfate hemi-hydrate phosphoric acid process for purification diminishes one of the advantages that low sulfate hemihydrate phosphoric acid processes have compared to other hemihydrate phosphoric acid processes.
One embodiment of the present invention permits the use of phosphoric acid produced from a low-sulfate hemihydrate phosphoric acid process in the manufacture of SPA without using additional sulfate ion, or with reduced consumption of sulfate ion during purification by aging and filtration.
Other processes for removing impurities are disclosed in U.S. Pat. Nos. 3,694,153; 4,053,564; and 4,082,836, wherein metallic impurities are extracted using a solvent which is relatively selective for the impurities. Other extraction processes are disclosed in U.S. Pat. No. 4,284,614 or Ore, U.S. Pat. No. 4,243,637 of Bradford et al., and U.S. Pat. No. 4,275,038 of Sikdar, Adams and Winterbottom. Certain embodiments of the invention can reduce costs when phosphoric acid is purified by such extraction processes.
In a preferred embodiment of the present invention, dihydrate acid is purified and blended with unpurified hemihydrate acid, which is more difficult to purify than dihydrate acid, to produce a stable product that can be concentrated and further processed to yield liquid fertilizer products having good storage and transportation keep properties, but are difficult to purify if solid impurities precipitate out.